> If room temperature and pressure superconducting or nuclear fusion became viable, your kids might also be unlikely to play around with them on their own.

I disagree. Kids will turn anything not too abstract into toys. An improvised radioactive sample (actually some neon indicator bulbs in a box) + Geiger counter brought by my parents = a version of a hide-and-seek game from my own past.

Also, a model maglev train is something so obvious that I am sure such toys will appear immediately once the technology becomes viable.